Semiconductor memory device with a three-dimensional stacked memory cell structure

ABSTRACT

A semiconductor memory device comprises: a semiconductor substrate; a plurality of memory units provided on the semiconductor substrate and each including a plurality of memory cells that are stacked; and a plurality of bit lines formed above each of a plurality of the memory units aligned in a column direction, an alignment pitch in a row direction of the plurality of bit lines being less than an alignment pitch in the row direction of the memory units, and an end of each of the memory units aligned in the column direction being connected to one of the plurality of bit lines formed above the plurality of the memory units aligned in the column direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation application claims the benefit of priority under 35U.S.C. § 120 from prior U.S. patent application Ser. No. 14/727,134,filed Jun. 1, 2015, which is a continuation of U.S. patent applicationSer. No. 14/307,154, filed Jun. 17, 2014, which is a continuation ofU.S. patent application Ser. No. 13/423,546, filed Mar. 19, 2012. Thisapplication is based upon and claims the benefit of priority from theprior Japanese Patent Application No. 2011-143500, filed on Jun. 28,2011. The entire contents of each of these applications are incorporatedherein by reference.

FIELD

The embodiments described herein relate to a semiconductor memory deviceand a method of control therein.

BACKGROUND Description of the Related Art

In recent years, several semiconductor memory devices having memorycells disposed three-dimensionally (stacked type semiconductor memorydevices) have been proposed to increase the degree of integration ofmemory.

In one known example of such a stacked type semiconductor memory device,a semiconductor pillar is formed extending in a perpendicular directionwith respect to a semiconductor substrate, and word lines disposed inmultiple layers in the perpendicular direction are connected to a sidesurface of that semiconductor pillar via a charge storage layer, therebyconfiguring a memory cell unit having memory cells connected in seriesin the perpendicular direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a semiconductormemory device according to a first embodiment.

FIG. 2 is a circuit diagram of part of a memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 3 is a perspective view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 4 is a cross-sectional view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 5 is a plan view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 6 is a cross-sectional view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 7 is a cross-sectional view of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 8 is a plan view of part of a memory cell array in a semiconductormemory device according to a comparative example.

FIG. 9 is a circuit diagram of a peripheral circuit in the semiconductormemory device according to the first embodiment.

FIG. 10 is a plan view of part of a memory cell array in a semiconductormemory device according to a second embodiment.

FIG. 11 is a cross-sectional view of part of the memory cell array inthe semiconductor memory device according to same embodiment.

FIG. 12 is a circuit diagram of part of the memory cell array in thesemiconductor memory device according to same embodiment.

FIG. 13 is a circuit diagram of part of a memory cell array in asemiconductor memory device according to a third embodiment.

FIG. 14 is a block diagram showing a configuration of the semiconductormemory device according to same embodiment.

FIG. 15 is a block diagram showing a configuration of part of aperipheral circuit in the semiconductor memory device according to sameembodiment.

FIG. 16 is a block diagram showing a schematic configuration of asemiconductor memory device according to a fourth embodiment.

FIG. 17 is a block diagram showing a schematic configuration of asemiconductor memory device according to a fifth embodiment.

FIG. 18 is a block diagram showing a schematic configuration of asemiconductor memory device according to a sixth embodiment.

DETAILED DESCRIPTION

A semiconductor memory device according to an embodiment comprises: asemiconductor substrate; a plurality of memory units provided on thesemiconductor substrate and each including a plurality of memory cellsthat are stacked; and a plurality of bit lines formed above each of aplurality of the memory units aligned in a column direction, analignment pitch in a row direction of the plurality of bit lines beingless than an alignment pitch in the row direction of the memory units,and an end of each of the memory units aligned in the column directionbeing connected to one of the plurality of bit lines formed above theplurality of the memory units aligned in the column direction.

Embodiments are described below with reference to the accompanyingdrawings.

First Embodiment Overall Configuration

First, an overall configuration of a semiconductor memory deviceaccording to a first embodiment is described with reference to FIG. 1.FIG. 1 is a block diagram showing the configuration of the semiconductormemory device according to the first embodiment.

As shown in FIG. 1, this semiconductor memory device includes a memorycell array 11 that comprises a plurality of memory blocks MB. Theplurality of memory blocks MB#0-#j (where j is a natural number) arealigned in a column direction (direction in which a bit line BLextends). All the memory blocks MB#0-#j share bit lines BL and a sourceline SL. The plurality of memory blocks MB#0-#j, in addition toincluding a plurality of the bit lines BL extending in the columndirection, include also a plurality of word lines WL and a plurality ofselect gate lines SGD and SGS extending in a row direction intersectingthe bit lines BL. The word lines WL are connected to a row decoder 12,and the select gate lines SGD and SGS are connected to a row decoder 13.Moreover, the bit lines BL are connected to a column decoder 15 via asense amplifier 14.

The row decoder 12 selects the word lines WL based on a row addressoutputted from an address pre-decoder 16, and applies a voltagegenerated by a word line driver 17 to, respectively, a selected wordline and an unselected word line.

The row decoder 13 selects a source side select gate line SGS and adrain side select gate line SGD corresponding to a memory unit MUactivated based on the row address outputted from the addresspre-decoder 16, and applies a gate voltage generated by a select gateline driver 18 to a selected source side select gate line SGS and drainside select gate line SGD.

The column decoder 15 decodes a column address signal outputted from theaddress pre-decoder 16 and performs input/output control of data. Thesense amplifier 14 senses and latches data of the bit line BL selectedby the column decoder 15. A controller 19 receives a signal forexecuting a read/write/erase operation and so on, from an addresscommand register not illustrated, and controls an internal voltagegenerating circuit not illustrated that generates various voltagesrequired in core operation, according to a certain sequence. Note that aperipheral circuit of the row decoders 12 and 13, the sense amplifier14, the column decoder 15, and so on, may be formed directly below thememory cell array 11.

Memory Cell Array

Next, a configuration of the memory cell array 11 is described withreference to FIGS. 2-7. FIG. 2 is a circuit diagram showing part of onememory block MB in the memory cell array 11.

As shown in FIG. 2, the memory block MB includes a plurality of memoryunits MU arranged in a matrix. One ends of the memory units MU arrangedin the column direction are connected alternately to two bit lines BLjand BLj+1 disposed along these memory units MU. The other ends of thememory units MU are commonly connected to the source line SL.

Each of the memory units MU includes a memory string MS, and a sourceside select transistor SSTr and drain side select transistor SDTrprovided at the two ends of the memory string MS. The memory string MSherein, which comprises, connected in series, a plurality of memorytransistors MTr1-MTr8 and a back gate transistor BTr connected betweenthe memory transistors MTr4 and MTr5, is configured having the sourceside select transistor SSTr and the drain side select transistor SDTrconnected respectively to the two ends of the memory string MS. Thememory transistors MTr1-MTr8 each have a MONOS structure, for example,that includes: a charge storage layer (for example, an insulating film)formed on a side surface of a semiconductor body via a gate insulatingfilm; an insulating film (insulating film having a higher permittivitythan the charge storage layer) formed on a side surface of the chargestorage layer; and a control gate formed on a side surface of thisinsulating film. The memory transistors MTr1-MTr8 each change theirthreshold voltage by storing a charge in their charge storage layer andthereby store information corresponding to this threshold voltage.

In the memory block MB, gates of memory transistors MTrj aligned in therow direction are commonly connected to a word line WLj extending in therow direction. In addition, in one memory block MB, word lines WLjconnected to corresponding memory transistors MTrj in each of the memoryunits MU are commonly connected. Moreover, gates of back gatetransistors BTr in the memory units MU are commonly connected to theback gate line BG.

In the memory block MB, gates of each of the drain side selecttransistors SDTr aligned in the row direction are commonly connected tothe drain side select gate line SGD extending in the row direction.Moreover, in the memory block MB, gates of each of the source sideselect transistors SSTr aligned in the row direction are commonlyconnected to the source side select gate line SGS extending in the rowdirection.

Next, a structure of the memory cell array 11 according to the firstembodiment is described with reference to FIGS. 3-7.

FIG. 3 is a perspective view showing a configuration of part of thememory cell array 11.

As shown in FIG. 3, the memory cell array 11 comprises a U-shapedcolumnar semiconductor layer 30 having both ends (source end and drainend) of the back gate transistor BTr extending upwardly in aperpendicular direction on a semiconductor substrate 20. Thesemiconductor layer 30 is disposed having its both ends (upper ends)aligned in the column direction, and is disposed in a plural matrix inthe column direction and the row direction. As shown in an enlarged viewof part of a cross-section of the semiconductor layer 30 in FIG. 4, thesemiconductor layer 30 is configured by a columnar semiconductor body31, a tunnel insulating layer 32 covering a side surface of thesemiconductor body 31, a charge storage layer 33, and a block insulatinglayer 34. Employable as the tunnel insulating layer 32 and the blockinsulating layer 34 is, for example, silicon oxide (SiO₂) or the like.Employable as the charge storage layer 33 is, for example, siliconnitride (SiN) or the like.

A back gate BG is disposed on the semiconductor substrate 20. The backgate transistor BTr is formed by this back gate BG and a folded part ofthe semiconductor layer 30. The folded part herein is described usingFIG. 3. The semiconductor layer 30 includes a first columnar portion30A, a second columnar portion 30B, and a folded portion 30C. The foldedpart refers to this 30C in FIG. 3.

Stacked around the first columnar portion 30A on one side, viainsulating layers, in order from the semiconductor substrate 20 side inan upwardly perpendicular direction, are conductive layers forming theword lines WL4, WL3, WL2, WL1, and the source side select gate line SGS.These conductive layers are connected to a side surface of thesemiconductor layer 30. Stacked around the second columnar portion 30Bon the other side, via insulating layers, in order from thesemiconductor substrate 20 side in an upwardly perpendicular direction,are conductive layers forming the word lines WL5, WL6, WL7, WL8, and thedrain side select gate line SGD. These conductive layers are connectedto a side surface of the semiconductor layer 30. As a result, the memorytransistors MTr1-8 are configured having the word lines WL1-8 as controlgates, and the U-shaped semiconductor body 31 as a channel body. Inaddition, the source side select gate line SGS, the drain side selectgate line SGD, and the back gate BG have the U-shaped semiconductorlayer 30 as a body to configure, respectively, the source side selectgate transistor SSTr, the drain side select gate transistor SDTr, andthe back gate transistor BTr.

That is, the memory transistors MTr1-8 and the back gate transistor BTrconfigure the memory string MS, having a stacking direction as a longdirection. Moreover, the memory string MS, the drain side select gatetransistor SDTr, and the source side select gate transistor SSTrconfigure the memory unit MU. A source side of the memory unit MU, thatis, one of the ends of the semiconductor layer 30, is connected to thesource line SL. A drain side of the memory unit MU, that is, the otherof the ends of the semiconductor layer 30, is connected to the bit lineBL via bit line contacts BC1 and BC2. The bit line BL and the bit linecontact BC2 are configured thinner than the semiconductor layer 30, and,in this example, a width of the bit line BL and a width of the bit linecontact BC2 are substantively equal.

FIG. 5 is a plan view of the memory cell array 11, and FIGS. 6 and 7 arecross-sectional views taken along the lines X-X′ and Y-Y′, respectively,of FIG. 5. As shown in FIG. 5, in the semiconductor memory deviceaccording to the present embodiment, the bit lines BL are arranged inthe row direction with a pitch P2 which is half of a pitch P1 of thememory units MU in the row direction. Moreover, memory units MU adjacentin the column direction are connected respectively to different bitlines BL. Therefore, two lines each of bit lines BL are extendedrespectively from one column group of memory units MU aligned in thecolumn direction.

As a comparative example, FIG. 8 shows a plan view of a generalsemiconductor memory device. In the comparative example, a pitch in therow direction of the bit lines BL and the memory units MU is configuredequal.

The memory unit MU has a memory hole formed deeply in the stackingdirection and has the semiconductor layer 30 formed having a chargestorage layer and insulating layer formed on its surface, henceminiaturization is not as easy as for the bit line BL. In contrast, thebit line BL can be lithographed by simple line-and-spacing, hencefurther miniaturization is easy by, for example, a sidewall transferprocess. Specifically, the bit line BL can be formed with a width ofabout 1/n times the width of the semiconductor layer 30 (where n is anatural number).

Focusing on this point, as shown in FIG. 5, the semiconductor memorydevice according to the present embodiment is configured having twolines of bit lines allocated to one column group of memory units MUaligned in the column direction.

More specifically, as shown in FIGS. 5-7, the bit line contact BC1 isconnected to an end of the semiconductor layer 30. A cross-section ofthis bit line contact BC1 is larger than a cross-section of thesemiconductor layer 30. A width in the row direction of the bit linecontact BC1 is about the same as a width of two bit lines. Further, thebit line BL to which the memory units MU adjacent in the columndirection are connected via the bit line contact BC2 differs for everyother one of the memory units MU. Specifically, a certain memory unit MUis connected to one of the two bit lines BL formed above the bit linecontact BC1. The memory unit MU adjacent to this memory unit MU in thecolumn direction is connected to the other of the bit lines BL. Toachieve such a method of connection, fellow bit line contacts BC2adjacent in the column direction are connected respectively to differentbit lines BL.

Peripheral Circuit

Next, configurations of the row decoders 12 and 13 in the semiconductormemory device according to the present embodiment are described withreference to FIG. 9. FIG. 9 is a circuit diagram for explaining theconfiguration of the row decoders 12 and 13 in the semiconductor memorydevice according to the present embodiment.

The row decoder 12 comprises: an address decoding circuit 121 forselecting a certain word line according to a row address outputted fromthe address pre-decoder 16; a level shifter 122 that generates a voltageon receiving input of a signal from the address decoding circuit 121;and a select transistor 123 that, on receiving input to its gate of thevoltage outputted from the level shifter 122, outputs to the selectedword line WL a voltage inputted from the word line driver 17.

The row decoder 13 comprises an address decoding circuit 131, a levelshifter 132, and a select transistor 133 similar to those in the rowdecoder 12, and further comprises: an inverter 134 for inverting avoltage outputted from the level shifter 132; and a select transistor135 that receives input to its gate of a signal from the inverter 134.

The configuration of the row decoder 13 is basically similar to that ofthe row decoder 12, but differs in that the select transistors 135 and133 cause a select gate line to be connected to one of either unselectedblock wiring SGunsel or selected block wiring SGSsel or SGDsel.

In the present embodiment, the row decoder 13 allows the selecttransistors SDTr and SSTr in memory units MU adjacent in the columndirection to be simultaneously selected. As a result, data can besimultaneously accessed from memory units MU in FIG. 2 adjacent in thecolumn direction via different bit lines BL. Therefore, the number ofbits in one page worth of data able to be read or written in one go canbe doubled compared to the comparative example shown in FIG. 8.

In such a way, the present embodiment allows the page length capable ofbeing read or written in a single read operation to be increased withoutincreasing circuit area. This results in performance of thesemiconductor memory device according to the present embodiment beingimproved.

Note that since the bit lines BL are connected to the sense amplifier14, there is a risk that increasing the number of bit lines leads to anincrease in circuit area. However, the semiconductor memory deviceaccording to the present embodiment is a semiconductor memory device ofthe so-called Pipe type employing the U-shaped semiconductor layer 30 asa channel body of the memory string MS. Hence, it is possible to bringthe wiring together above the memory cell array 11 and form the senseamplifier 14 under the memory cell array 11. Therefore, circuit area isdetermined by area of the memory cell array 11, and it can be preventedthat circuit area increases to be larger than area of the memory cellarray 11.

Second Embodiment

Next, a configuration of a memory cell array 11 according to a secondembodiment is described with reference to FIGS. 10-12. FIG. 10 is a planview showing a configuration of part of the memory cell array; FIG. 11is a cross-sectional view taken along the line Y-Y′ of the plan viewshown in FIG. 10; and FIG. 12 is a circuit diagram showing aconfiguration of part of the memory cell array. Note that identicalsymbols are assigned to configurations identical to those in the firstembodiment, and descriptions thereof are omitted.

In the first embodiment, the bit lines BL are arranged with a pitch half(½) that of the memory units MU, but in the present embodiment, the bitlines BL are arranged with a pitch a third (⅓) that of the memory unitsMU. In addition, the bit lines BL are connected to every second memorystring aligned in the bit line BL direction. That is, when m is assumedto be a natural number, a 3m-th one of the memory units MU aligned inthe bit line BL direction is connected to the bit line BL1, a (3m+1)-thone of the memory units MU aligned in the bit line BL direction isconnected to the bit line BL2, and a (3m+2)-th one of the memory unitsMU aligned in the bit line BL direction is connected to the bit lineBL3.

Such a configuration causes page length to become 1.5 times larger thanthat in the first embodiment, and three times larger than that in aconventional device, thereby allowing read/write speed to be furtherimproved.

Note that in the present embodiment, the bit lines BL are arranged witha pitch a third (⅓) that of the memory units MU, but the presentinvention is not limited to this configuration, and the bit lines BL maybe arranged with a pitch that is any integer fraction that of the memorystrings MS. Moreover, bit lines BL having an integer fraction pitch mayalso be arranged in some of the memory units MU only.

Third Embodiment

Next, a semiconductor memory device according to a third embodiment isdescribed with reference to FIGS. 13-15. FIG. 13 is a circuit diagramshowing a configuration of part of a memory cell array 11 in thesemiconductor memory device according to the present embodiment; FIG. 14is a block diagram showing overall configuration; and FIG. 15 is circuitdiagram showing a configuration of a peripheral circuit.

In the first embodiment, select gate lines SGS and SGD are independentlyconnected to gates of the select gate transistors SSTr and SDTr,respectively, in a plurality of memory units MU adjacent in the columndirection. In the present embodiment, as shown in FIG. 13, the memoryunits MU adjacent in the column direction configure pairs, and a selectgate line SGS and SGD are commonly connected to the two memory units MUconfiguring such a pair.

Specifically, for example, the drain side select gate lines SGD<0> andSGD<1> in the first embodiment shown in FIG. 2 are commonly connected inthe third embodiment to configure a drain side select gate line SGD<0>′.Similarly, the source side select gate lines SGS<0> and SGS<1> in thefirst embodiment shown in FIG. 2 are commonly connected in the thirdembodiment to configure a source side select gate line SGS<0>′ in thepresent embodiment.

As shown in FIGS. 14 and 15, such a configuration enables the number ofselect gate lines connecting the memory cell array 11 and the rowdecoder 13 to be halved. Moreover, as shown in FIG. 15, reducing thenumber of select gate lines also allows circuit area of the row decoder13 to be suppressed to half (½).

Such a configuration is made possible by having the memory units MUadjacent in the column direction connected respectively to different bitlines BL. Therefore, when the bit lines BL are disposed with a pitch athird (⅓) that of the memory units MU as in the second embodiment, threeselect gate lines may be commonly connected. That is, how many selectgate lines are commonly connected may be determined appropriately by adispositional relationship of the memory units MU and bit lines BL.

Fourth Embodiment

Next, a fourth embodiment is described with reference to FIG. 16. FIG.16 is a block diagram showing a schematic configuration of thesemiconductor memory device according to the present embodiment.

In the present embodiment, two lines of bit lines BL are allocated toone column group of memory units MU aligned in the column direction,similarly to in the first embodiment, but the same number of senseamplifiers are provided as there are column groups of memory units MU.In this case, since one sense amplifier 14 is used alternately by twolines of bit lines BL, a selecting circuit SEL is provided between thebit lines BL and the sense amplifier 14.

In the case of this embodiment, area of the sense amplifier can besuppressed to an area similar to that in a conventional device. In orderto read or write data alternately to adjacent bit lines, each of thememory units MU is independently supplied with, respectively, selectgate lines SGS and SGD.

Fifth Embodiment

FIG. 17 is a block diagram showing a schematic configuration of asemiconductor memory device according to a fifth embodiment. A basicconfiguration of the present embodiment is similar to that of the firstembodiment, but differs in that a differential type sense amplifier isused as the sense amplifier 14. In the present embodiment, a pair cellis configured by a pair of corresponding memory transistors MTr inmemory units MU adjacent in the column direction, and this pair cellstores data that differs logically one from another. In this case, apair of data is read from adjacent bit lines BL and differentialdetection is performed by the sense amplifier 14.

This embodiment allows configuration of a memory resistant to noise,disturbance, and the like.

Sixth Embodiment

FIG. 18 is a block diagram showing a schematic configuration of asemiconductor memory device according to a sixth embodiment. In thepresent embodiment, one of two bit lines BL is connected to the senseamplifier 14, and the other of the two bit lines BL is grounded and usedas a shield line.

The present embodiment, while setting the number of bits of a page thatare read at one time to be the same as in a conventional device, allowsevery other bit line BL to be used as a shield, hence enables evengreater stability of data read to be achieved.

Note that the above-described embodiments 1-6 relate to a Pipe typesemiconductor memory device, but the present invention may of coursealso be applied to an I type semiconductor memory device using a pillarsemiconductor as a channel body of the memory unit MU. Circuitconfigurations and so on indicated in the embodiments described abovemay of course also be changed.

Other

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A semiconductor memory device, comprising: first,second, and third bit lines extending in a first direction, the thirdbit line being adjacent to the first bit line and the second bit line ina second direction, the second direction crossing the first direction; afirst memory unit including a first memory cell and a second memory cellstacked in a third direction, the third direction crossing the firstdirection and the second direction; a second memory unit including athird memory cell and a fourth memory cell stacked in the thirddirection; a third memory unit including a fifth memory cell and a sixthmemory cell stacked in the third direction; a first bit line contactconnected to the first bit line and the first memory unit; a second bitline contact connected to the second bit line and the second memoryunit, the first bit line contact and the second bit line contact beingprovided along a first line crossing the third direction; a third bitline contact connected to the third bit line and the third memory unit,the first bit line contact and the third bit line contact being providedalong a second line crossing the third direction and the first line; anda first conductive layer connected to a gate of the first memory cell, agate of the third memory cell, and a gate of the fifth memory cell. 2.The semiconductor memory device according to claim 1, furthercomprising: a fourth bit line, the second bit line being adjacent to thefourth bit line in the second direction; a fourth memory unit includinga seventh memory cell and a eighth memory cell; a fourth bit linecontact connected to the fourth bit line and the fourth memory unit, thethird bit line contact and the fourth bit line contact being providedalong a third line parallel to the first line, and the first conductivelayer being connected to a gate of the seventh memory cell.
 3. Thesemiconductor memory device according to claim 2, wherein the firstline, the second line and the third line is provided in the same plane.4. The semiconductor memory device according to claim 1, wherein a firstdistance is provided between the first bit line contact and the secondbit line contact, a second distance is provided between the first bitline and the third bit line, and the first distance is larger than thesecond distance.
 5. The semiconductor memory device according to claim1, wherein at least one of the first to third memory units comprises: asemiconductor body extending in the third direction; and a chargestorage layer provided between the semiconductor body and the firstconductive layer.
 6. The semiconductor memory device according to claim1, wherein a read or write operation is performed to the first memorycell and the fifth memory cell simultaneously.
 7. The semiconductormemory device according to claim 1, further comprising: a senseamplifier connected selectively or simultaneously to the first bit lineand the third bit line.
 8. The semiconductor memory device according toclaim 7, wherein a differential type sense amplifier is used as thesense amplifier.
 9. The semiconductor memory device according to claim1, further comprising a sense amplifier, wherein the sense amplifier isconnected electrically to the first bit line in a read or writeoperation and the third bit line is employed as a shield line in theread or write operation.
 10. The semiconductor memory device accordingto claim 1, wherein at least one of the first to third memory unitscomprises: a semiconductor body including a first portion, a secondportion and a third portion, the first portion and the third portionextending in the third direction, the second portion being providedbetween a substrate and the first portion and between a substrate andthe third portion, the second portion being connected to an end of thefirst portion and an end of the third portion.
 11. The semiconductormemory device according to claim 1, wherein at least one of the first tothird memory units comprises: a U-shaped semiconductor body; and acharge storage layer provided between the semiconductor body and thefirst conductive layer.